APPENDIX B. NOMENCLATURE (CONTINUED)
Figure B-2. Definition of Local Impingement Efficiency Parameter, ȕ.
Lower horn angle: The angle of the lower horn of a glaze ice shape, șlower, calculated with the polar direction from the wing chord plane (figure B-3).
Figure B-3. Definition of Horn Maximum Thickness Angle (Reference B1).
Ludlam limit: The value of LWC when the maximum rate of freezing can occur on a surface for a given combination of surface temperature, airspeed, altitude, and drop collection efficiency.
Mean effective diameter (MED): The drop diameter which divides the total water volume present in the drop distribution in half. So, half the water volume will be in larger drops and half the volume in smaller drops. The value is based on an assumed drop size distribution.
B-6
APPENDIX B. NOMENCLATURE (CONTINUED)
Median volume diameter (MVD): The drop diameter that divides the total water volume present in the drop distribution in half. So, half the water volume will be in larger drops and half the volume in smaller drops. The value is obtained by actual drop size measurements.
Mixed ice: A simultaneous appearance or a combination of rime and glaze ice characteristics.
Accurate identification of mixed ice from the cockpit may be difficult since the clarity, color, and shape of the ice will be a mixture of rime and glaze characteristics.
Mixed-phase icing conditions (mixed conditions): Partially glaciated clouds at an ambient temperature below 0°C. The clouds contain ice crystals and supercooled liquid water drops.
Moderate icing: A descriptor used operationally by flight crews to report encountered icing intensity to traffic control. The rate of ice buildup requires frequent cycling of manual deicing systems to minimize ice accretions on the airframe. A representative accretion rate for reference purposes is 1 to 3 inches (2.5 to 7.5 cm) per hour on the outer wing. The pilot should consider exiting the condition as soon as possible.
Monitored surface: The surface of concern regarding ice hazard (for example, the leading edge of a wing).
NACA inlet: A flush mounted divergent channel that creates vortices that entrain air into the inlet. Sometimes it is called a submerged inlet. NACA developed these inlets in the late 1940s and early 1950s.
Natural icing flight tests: Flight tests performed in icing conditions that occur in a naturally formed cloud.
Navier-Stokes: The name given to the equations of motion for flow of a compressible, viscous fluid. A Navier-Stokes solver is typically a computer code algorithm that provides a numerical solution of the Navier-Stokes equations. A Navier-Stokes computer code is a computational fluid-dynamics code that contains a Navier-Stokes solver and computes the flow of a compressible or incompressible, viscous fluid. Accurate determination of the motion of a fluid (air) at high Reynolds number becomes important in the thin layer of air bounding the surface of a body. As the flow of boundary-layer transitions from laminar to turbulent, and then to separation, the viscosity and compressibility of the fluid must be addressed. Solutions for the flow field in the boundary layer may be approximate. Those using solvers other than a full solution of the Navier-Stokes equations are further limited by additional approximations used to avoid solving the full equations. These lesser solvers typically average the Reynolds stresses across the turbulent boundary layers by using empirically based approximations. The robustness of the various approximations varies. Some provide reasonable information as the intensity of the boundary-layer turbulence increases. Using these approximations is questionable with separated boundary-layer flow. The types of flow conditions for which the empirical data are valid, therefore, limit these approximations. Full numeric solution of the Navier-Stokes equations is difficult and costly. The five equations must be solved simultaneously for a multitude of very small finite volumes for surface area of high curvature to obtain good accuracy. A common practice is to limit the volume in which the full equations are solved to the region next to the B-7
APPENDIX B. NOMENCLATURE (CONTINUED)
surface. The less complex far field flow solutions are computed using simplified solutions of the equations. The far field flow viscosity and compressibility effects are less important. Attention must be given to the interface between the two flow solutions.
Panel code: A computational fluid-dynamics code that solves for the average flow over finite panels that define a body surface.
Potential flow: Given a flow field specified by a vector function, V, if V can be written as:
V = M (Eqn. B-1) for some scalar function M, then the flow is referred to as a potential flow (and M is called the potential function.) Inviscid, irrotational flows are potential flows. For example, potential flow equations can be used to calculate the influence of a flow phenomenon (for example, a source or sink) on the velocity potential elsewhere in an irrotational, nonviscous fluid flow field. The fluid may be incompressible or compressible. Potential flow codes are the simplest of the flowfield solvers. However, they do not address the issues of the real fluid rotational flow and viscosity.
The flowfield solutions produced by the potential flow equation provide useful aerodynamic information for problems where flow rotation and viscosity effects are insignificant.
Potential icing conditions: Atmospheric icing conditions that airframe manufacturers typically define in terms of temperature and visible moisture that may result in aircraft ice accretion on the ground or in flight. The potential icing conditions are typically defined in the airplane flight manual or in the airplane operation manual.
Preactivation ice: Protected surface ice accretion that occurs before the ice protection system becomes fully effective.
Protected surface: A surface containing ice protection. The protected surface is typically located at the surface’s leading edge.
Protection surface: The active surface of an ice protection system (for example, the surface of a deicing boot or thermal ice protection system).
Reference surface: The observed (directly or indirectly) surface used as a reference for ice presence on the monitored surface. Ice presence on the reference surface must occur before —or coincidentally with—ice presence on the monitored surface. Examples of reference surfaces include windshield wiper posts, ice evidence probes, propeller spinners, and the metric sensor surface of ice detectors. The reference surface may also be the monitored surface.
Residual ice: Ice that remains on a protected surface immediately after deicing system actuation.
Reynolds averaged Navier-Stokes (RANS): Navier-Stokes solver schemes that approximate the momentum changes due to turbulence (Reynolds stresses). The momentum changes are calculated in the boundary-layer wake through a set of equations.
B-8
APPENDIX B. NOMENCLATURE (CONTINUED)
Reynolds number: A dimensionless parameter that is the ratio of the inertia force to viscous force for a fluid. It is calculated according to the formula:
Re = ȡaVL/ȝ (Eqn. B-2) V is a characteristic velocity, ȡa is the density of the fluid, ȝ is the viscosity of fluid, and L is a characteristic length. Flow within the boundary layer of a surface correlates strongly with the Reynolds number of that flow. The Reynolds number also correlates with the transition between laminar and turbulent flow in the boundary layer. This occurs at a critical Reynolds number for a flat plate. Reynolds number parameters also characterize the behavior of a separated boundary layer. The Reynolds number is a scaling parameter for fluid flow over a surface since it controls transition and separation of the flow across the surface. To get similar full-scale aerodynamic parameter estimates (surface pressures) through testing of scaled models, the Reynolds number of the scaled model test should be similar to that of the full-scale aircraft in flight. Testing techniques, such as early tripping of the laminar boundary layer (that occur at low Reynolds numbers) to simulate regions of full-scale turbulent boundary-layer flow, have been used to avoid the expense of testing at Reynolds numbers approaching full-scale. Typically, testing at high Reynolds numbers requires the expensive use of pressure or cryogenic wind tunnels.
Rime ice: A rough, milky, opaque ice formed by the rapid freezing of supercooled drops after they strike the aircraft. The rapid freezing results in trapped air. The trapped air gives the ice its opaque appearance and makes it porous and brittle. Rime ice typically accretes along the stagnation line of an airfoil and is more regular in shape and conforms more to the airfoil than glaze ice. Crew are more likely to assess the ice shape, rather than the clarity or color of the ice accurately from the cockpit.
Runback ice: Ice that forms from the freezing or refreezing of water leaving protected surfaces and running back to unprotected surfaces.
Running wet: Defines heat requirements for running wet anti-icing that are based on maintaining an above-freezing surface temperature. This allows some of the impinging water to run back and freeze aft of the heated area or off the surface.
Running wet system: Any anti-icing system that supplies enough heat to prevent impinging water drops from freezing on the heated surface. A running wet system does not supply enough heat for complete evaporation.
s/c: The surface distance, normalized by chord length, aft of the leading edge of the surface (airfoil).
Separated flow: A flow condition in which the flow is no longer attached to the surface. This phenomenon is associated with vortex formation and large energy losses in the flow. Separated flow typically results in losses in lift, increased drag, and reduced control effectiveness of lifting surfaces.
B-9
APPENDIX B. NOMENCLATURE (CONTINUED)
Severe icing: A descriptor used operationally by flight crews reporting encountered icing intensity to traffic control. The rate of ice buildup results in the inability of the ice protection systems to remove the buildup of ice satisfactorily. Also, ice builds up in locations not normally prone to icing, such as areas aft of protected surfaces and any other areas identified by the manufacturer. Immediate exit from the condition is necessary.
Shedding: Ice shedding is the act of separating or breaking away accreted ice from an aircraft part. Ice may shed by passive means for any aircraft surface (for example, by the natural aerodynamic or centrifugal forces) or by active means for protected surfaces of the aircraft (for example, by a deicing system).
Simulated ice shapes: Ice shapes made of wood, epoxy, or other materials by any construction technique. Simulated or artificial ice shapes can be designed and manufactured to reproduce ice shapes accumulated during simulated icing conditions. Simulated critical ice shapes may be tested during certification of ice protection systems. (See critical ice shapes and validation.)
Simulated icing: The process of creating simulated ice, for example, accumulating ice on an aircraft or aircraft surface by using a spray array, in an icing tunnel or behind an icing tanker.
Supercooled drops: Water drops that remain unfrozen at temperatures below 0qC. Supercooled drops exist in clouds, freezing drizzle, and freezing rain in the atmosphere. These drops may impinge and freeze after contact on aircraft surfaces.
Supercooled large drops (SLD): Liquid drop with diameters greater than 0.05 mm at temperatures less than 0qC, i.e., freezing rain or freezing drizzle.
Total water catch efficiency (E): The total amount of water or ice that impinges on the aircraft surface. It is the integrated value of the local catch. For a two-dimensional (2D) body (for example, on an airfoil) the total catch is more conveniently expressed as a unit span. An alternative (and equivalent) measurement (for a 2D case) is the weighted average of the water catch efficiencies shown in equation B-3.
Eds³ E (Eqn. B-3) ds³ Upper horn angle: The angle of the upper horn of a glaze ice shape, șupper, calculated with the polar direction from the wing chord plane (figure B-3).
Validation: The process that confirms that a computer code is functioning correctly;
documentation and code version control meet the standards of the validating organization; and that predicted ice shapes match accepted experimental data, according to accepted validation standards (within established tolerances).
Verification: The process of determining that implementation of algorithms (for example, a computer code) accurately represents the developer’s conceptual description of a problem B-10
APPENDIX B. NOMENCLATURE (CONTINUED)
solution. The code should accurately implement the mathematical rules or procedures used to solve the problem.
Water catch: The mass of water captured between the upper and lower impingement limits during a specified interval of time.
Water catch efficiency (ȕ): The ratio of actual water drops mass flux at the surface to the water drops mass flux in the freestream when water drop paths are straight lines. It is also known as the collection efficiency, impingement efficiency, or local impingement efficiency.
Weber number: A dimensionless parameter that is the ratio of the inertia of air to the surface tension force at the air/water interface. It could also be the ratio of these parameters for any two fluids. For the interface of air and water, the Weber number ( We) is:
Wel = UwV 2L/Vw/a (Eqn. B-4) V is a characteristic velocity, Uw is the density of water, Vw/a is the water surface tension, and L is a characteristic length (for example, water film thickness and drop diameter). Similar to Reynolds number, the Weber number is a scaling parameter for liquid fluids and is used to explore liquid flow behavior or ice accretion by using scaled models or when testing with scaled icing cloud drops. Typically, it is not possible to hold the Reynolds number and Weber number constant for a scaled test. Resolution of this dilemma typically results in a compromise with favor given to one or the other parameter. This depends on the scaling parameter’s influence on the desired test results. For some cases of ice accretion, determining the scaled freestream velocity by holding model-scale and full-scale Weber number (based on leading edge water film thickness) constant and holding the freestream temperature the same has resulted in satisfying comparisons between full and model scale ice shapes. Alternatively, the model scale freestream velocity may be determined by averaging the values determined by holding the Weber and Reynolds number constant between full scale and model scale.
B.2 ACRONYMS AND SYMBOLS.
ac Aerodynamic center of lift (sectional) A, Ac Accumulation parameter, [V(LWC)t]/[ȡiceL] A/C Aircraft AC Advisory Circular AD Airworthiness Directive AFIDS Advisory flight ice detector system AFM Airplane Flight Manual AIAA American Institute of Aeronautics and Astronautics AGL Above ground level AOA Angle-of-attack, degrees APMS Aerodynamic Performance Monitoring System Appendix C Appendix C of 14 CFR Part 25 APU Auxiliary power unit B-11
APPENDIX B. NOMENCLATURE (CONTINUED)
ARP Aerospace Recommended Practice AS Aerospace Specification ASIC Application specific integrated circuits ASL Altitude with respect to sea level BITE Built-in test equipment b Relative heat factor, [LWCȕ̦0cp,ws]/hc C Celsius c Chord cd Drag coefficient (sectional) cl Lift coefficient (sectional) cm Pitching-moment coefficient (sectional) cp Specific heat of air cp,ws Specific heat of water at the surface temperature CP Pressure coefficient CAR Civil Air Regulations CIRA Centro Italiano Ricerche Aerospaziali CFD Computational fluid dynamics CFR Code of Federal Regulations d Twice the airfoil’s leading edge radius DER Designated engineering representative DES Detached eddy simulation DNS Direct numerical simulation E Total water catch efficiency ECDS Eddy current deicing system EEDS Electro-expulsive deicing system EIDI Electro-impulse deicing system Eqn. Equation ETOPS Extended range operation with twin-engine airplanes F Fahrenheit FAA Federal Aviation Administration, United States FIDS Flight ice detection systems FMEA Failure modes and effects analysis Ft Feet FZDZ Freezing drizzle FZRA Freezing rain g Gram(s) h Height hf Water film thickness hc Convective heat-transfer coefficient hG Gas-phase mass-transfer coefficient HE Horizontal extent ICTS Ice-contaminated tailplane stall IFC Instrument flight conditions IFR Instrument flight rules IMC Instrument meteorological conditions B-12
APPENDIX B. NOMENCLATURE (CONTINUED)
In Inch(es) IPS Ice protection system KOEL Kind of equipment list K Drop inertia parameter, [ ȡwMVD2V]/[18dȝ], (c is sometimes used rather than d in the denominator.)
K0 Modified drop inertia parameter, [O/OStokes]K k Height of the surface roughness or protuberance k/c Height of the surface roughness or protuberance normalized by surface chord kt Knots LES Large eddy simulation LWC Liquid water content M Mach number m Meter(s) MED Mean effective diameter Microns Micrometer min Minute mm Millimeter(s) MMEL Master minimum equipment list mph Miles per hour MVD Median volume diameter n Freezing fraction, [cp,ws/ ȁf][ij + ș/b] NACA National Advisory Committee for Aeronautics NASA National Aeronautics and Space Administration NASA-GRC National Aeronautics and Space Administration Glenn Research Center NLF Natural laminar flow nm Nautical miles OAT Outside ambient temperature p Static pressure PFIDS Primary flight ice detector system (also known as PIIDS – primary in-flight ice detection system) PLD Programmable logic devices pw Vapor pressure of water in the atmosphere pww Vapor pressure of water at the icing surface QA Heat available QR Heat required r Recovery factor rpm Revolutions per minute RANS Reynolds-averaged Navier-Stokes R, Re Reynolds number RFM Rotorcraft Flight Manual RTCA Radio Technical Commission for Aeronautics, Inc.
SAE Society of Automotive Engineers s Surface length s, sec Second(s) SFAR Special Federal Aviation Regulations B-13
APPENDIX B. NOMENCLATURE (CONTINUED) SLD Supercooled large drops t Time tf Freezing temperature ts Surface temperature tst Static temperature T Temperature TS Static air temperature TAS True airspeed TN Technical note V Velocity VD Dive velocity VMO Maximum operating velocity VNE Never exceed velocity VFR Visual flight rules W Ice accretion rate We Weber number Wel Weber number based on surface length Weh Weber number based on water-film thickness and water properties Wdetector Ice accretion rate of the ice detector sensor Wengine Ice accretion rate of the engine Wwing Ice accretion rate of the wing x/c Longitudinal distance aft of the surface’s (airfoil’s) leading edge normalized by the chord of the surface (airfoil) y/c Vertical distance above the surface’s (airfoil’s) reference line normalized by the chord of the surface (airfoil) 2D Two-dimensional 3D Three-dimensional D Angle-of-attack, degree(s) ȕ Local water catch efficiency ȕ̦0 Collection efficiency at the stagnation line ȁ Leading edge sweep angle ȁf Latent heat of freezing ȁv Latent heat of condensation Ȝ Drop range ȜStokes Drop range if Stokes Law applies ij Water drop energy transfer parameter, tf – tst – V2/[2cp,ws] º Degree(s) ȡa Density of air ȡi Density of ice ȡw Density of water ș Air-energy transfer parameter, (ts – tst – [rV2]/[2cp,ws]) + ([hG/hc][pww - pw]) ȁv ıw/a Surface tension of water divided by surface tension of air ȝ Absolute viscosity of air ȝm Micrometer, micron § Section of the Code of Federal Regulations B-14
APPENDIX B. NOMENCLATURE (CONTINUED)
s Inch(s) > Greater than < Less than Divergence B.3 REFERENCES.
B1. “Ice Accretion and Droplet Impingement Codes,” SAE ARP5903.
B-15 (and B-16)
APPENIDX C. BACKGROUND OF ICE PROTECTION REGULATORY
DEVELOPMENT The following background information is provided to determine the certification basis for approved aircraft ice protection systems, as needed for STC applications, and for tracking icing requirement changes.
C.1 TRANSPORT CATEGORY AIRPLANES (14 CFR PART 25).
The FAA certified airplanes under the Civil Air Regulations (CAR) part 04 until 1953. Civil Air Regulations § 4a.5814 required that if manufacturers installed deicing boots, then they had to provide positive means to deflate all wing boots. There were no other references to an airplane ice protection system (IPS) in CAR part 04.
The FAA codified Part 4b of the CAR on December 31, 1953. The requirement for positive means of deflating deicing boots was incorporated, with change, in CAR § 4b.640. CAR § 4b.640 stated that:
“When an ice protection system is installed, it shall be of an approved type. If pneumatic boots are used, at least two independent sources of power and a positive means for the deflation of the boots shall be provided.”
CAR § 4b.351(b)(ii) provided requirements for pilot compartment vision in icing conditions.
CAR § 4b.406 provided propeller-deicing requirements. CAR § 4b.461 provided for preventing and eliminating ice buildups in engine air induction systems. CAR § 4b.612(a)(5) required that the airspeed indicating system have a heated pitot tube or equivalent means of preventing malfunctioning due to icing.
CAR Amendment 4b-2, effective August 25, 1955, introduced icing envelopes similar to the current 14 CFR part 25, Appendix C. The graphs added by CAR Amendment 4b-2 (figures 4b-24a, 4b-24b, 4b-24c, 4b-25a, 4b-25b, and 4b-25c) were identical in substance and format to the current 14 CFR part 25, Appendix C envelopes with a few exceptions. These envelopes described the liquid water content (LWC), the mean effective diameter (MED) of drops, the temperature, and horizontal and vertical extent of the supercooled icing cloud environment. In the figures introduced by CAR Amendment 4b-2, the units of the distances shown on the graphs were expressed in statute miles instead of nautical miles. If the correction for the difference in value between nautical and statute miles is made, the LWC is identical between the CAR Amendment 4b-2 figures and the current 14 CFR part 25, Appendix C envelopes.
There are two significant differences between the CAR Amendment 4b-2 envelopes and the current 14 CFR part 25, Appendix C. Originally, the minimum MED in the intermittent maximum conditions was 20 micrometers versus the current MED of 15 micrometers. The LWC-versus-distance factor chart minimum cloud horizontal extent was 1.5 statute miles, compared with the current 0.26 nautical miles (0.3 statute miles). The selected icing cloud envelopes were considered to be important for the design of thermal ice protection systems on C-1